1. Field of the Invention
The present invention relates to an optical head device for recording information to, or reproducing or erasing information from, an information memory medium, for example, an optical disk or optical card. The present invention also relates to an optical information processing apparatus, and an inclination angle detection apparatus for detecting an angle made by a beam collected by a light collection system in an optical information processing apparatus and an information memory medium.
2. Description of the Related Art
Optical memory technologies which use optical disks or optical cards as high density, large capacity memory media are used in progressively wider fields, for example, in digital audio disks, video disks, document file disks and data files. By such optical memory technologies, information is recorded to, or reproduced from, an optical disk with sufficiently high precision and satisfactory reliability through a light beam which is focused to have a microscopic diameter. The performance of a recording and reproduction apparatus using the optical memory technologies significantly relies on the optical system.
Exemplary basic functions of the optical head device, which is a main part of the optical system, are rough classified into:
(1) light collection in order to form a smallest possible light spot only limited by the diffraction;
(2) focusing and tracking control of the optical system, and reproduction of information signals; and
(3) erasing and writing of information signals by collected light.
These functions are realized by a combination of various optical systems and a light detector of a photoelectric conversion system.
As a first conventional example comparative to the present invention, a conventional optical head device will be described with reference to FIG. 42. FIG. 42 is a schematic view of an optical system of the conventional optical head device. In the optical head device shown in FIG. 42, focusing is performed by the non-point aberration method and tracking is performed by the push-pull method and the phase contrasting method.
The optical head device shown in FIG. 42 operates in the following manner.
Light emitted by a semiconductor laser 101 as a light source is reflected by a plane-parallel beam splitter 102 and collimated by a collimator lens 103, which is included in a light collection system. The light is then collected by an objective lens 104 which is also included in the light collection system, and collected on an information layer 108 of an optical disk 105, which is an information memory medium. An actuator 107 moves the objective lens 104 and a holding device 106 in accordance with fluctuations or decentration of the optical disk 105.
The light is then diffracted and reflected by the information layer 108 of the optical disk 105 to be reflected light 108a. The reflected light 108a is converged by the collimator lens 103. The reflected light 108a is then provided with an non-point aberration when passing through the plane-parallel beam splitter 102. The light provided with the non-point aberration is received by a light detector 150. The above-described elements in the optical system shown in FIG. 42 are arranged so that, when a focal point F0 of the light from the objective lens 104 is on the information layer 108, a light detecting surface of the light detector 150 is in the least circle of confusion of the converged light provided with the non-point aberration.
FIG. 43A shows a pattern of a light detection area of the light detector 150 and the shape of a cross section of the reflected light 108a detected by the light detector 150. The light detector 150 includes four light detection areas 251 through 254. Signals obtained in accordance with the amount of light received by the light detection areas 251 through 254 are referred to herein as s1 through s4. Although an operation circuit for generating a tracking error signal is not shown, a tracking error signal TE1 is generated according to expression (1).TE1=(s1+s4)−(s2+s3)  (1)
By the phase contrasting method, a tracking error signal TE2 is obtained by comparing the phase of a sum signal of s1 and s3 and the phase of a sum signal of s2 and s4.
A focusing error FE signal by the non-point aberration method is generated according to expression (2).FE=(s1+s3)−(s2+s4)  (2)
When the information layer 108 of the optical disk 105 is distanced from the objective lens 104 so as to be beyond the focal point F0 of the light from the objective lens 104, the cross section of the reflected light 108a detected by the light detector 150 is as shown in FIG. 43B. When the information layer 108 of the optical disk 105 approaches the objective lens 104 so as to be between the objective lens 104 and the focal point F0 of the light from the objective lens 104, the cross section of the reflected light 108a detected b the light detector 150 is as shown in FIG. 43C.
An RF signal, which is an information reproduction signal, is a sum of the signals s1 through s4 obtained from all the light detection areas and thus is generated according to expression (3).RF=s1+s2+s3+s4  (3)
The conventional optical head device described above have the following problems.
(1) The tracking error signal is generated by a differential signal which indicates the difference between the signals respectively obtained from the two light detection areas defined by simply equally dividing the light detection surface (aperture) of the light detector 150 into two by a central line of the aperture. In such a structure, the light is incident off the track or tracking is not stably controlled when an aberration occurs due to an inclination of the objective lens 104 and/or the optical disk 105 (tilt), or when the objective lens moves in a direction perpendicular to the tracks with respect to the optical axis in accordance with the decentration of the optical disk 105.
(2) When the focal point of the light from the objective lens 104 scans the position off the track in which the information to be reproduced is stored, if a reproduction signal is generated by a signal indicating the difference between the signals respectively obtained from the two light detection areas defined by simply equally dividing the aperture of the light detector 150 into two by a central line of the aperture, a sufficient margin with respect to the disturbance cannot be secured.
Regarding an inclination angle detection apparatus for detecting an inclination of a beam collected by a light collection system in an optical information processing apparatus with respect to the information memory device, various structures have been proposed in order to accurately read information from, and write information to, the information memory device.
As a second conventional example comparative to the present invention, a conventional inclination detection apparatus will be described with reference to FIG. 44. FIG. 44 is a schematic view of an inclination detection apparatus. The inclination detection apparatus shown in FIG. 44 operates in the following manner.
A linearly polarized scattering beam 70 emitted from a semiconductor laser 101 as a light source is collimated by a collimator lens 103 and then is incident on a polarizing beam splitter 130. Next, the beam 70 is transmitted through the polarizing beam splitter 130 and then through a ¼-wave plate 122 to be converted into a circularly polarized beam. The circularly polarized beam is collected on an optical disk 105 as an information memory medium by an objective lens 104.
FIG. 45 shows a structure of the optical disk 105. In FIG. 45, Gn−1, Gn, Gn+1, . . . each represent a guide groove. Information is stored in the guide grooves as a mark or a space. Accordingly, tracks Tn−1, Tn, Tn+1, . . . for storing information correspond to the guide grooves Gn−1, Gn, Gn+1, . . . . Also in FIG. 45, Gp represents a space between two adjacent guiding grooves (i.e., cycle of the grooves), and tp represents a space between two adjacent tracks (i.e., cycle of the tracks). The values of Gp and tp are equal to each other.
The beam 70 which is reflected and diffracted by the optical disk 105 is again transmitted through the objective lens 104 and then through the ¼-wave plate 122 to be converted into a linearly polarized beam which runs in a direction perpendicular to the direction of the light emitted from the semiconductor laser 101. The beam 70 is then entirely reflected by the polarizing beam splitter 130 and converted into a converged beam (still indicated by reference numeral 70) by a detection lens 133. The converged beam 70 is transmitted to the planar polarizing plate 134 and received by a light detector 158. The beam 70 is provided with a non-point aberration for focusing error detection when passing through the planar polarizing plate 14. The beam 70 received by the light detector 158 is converted into an electric signal in accordance with the light amount thereof.
In this specification, in the case where the optical disk is a ROM disk, a mark indicates a pit, and a space indicates a plane part. In the case where the optical disk indicates a phase-change memory medium, a mark indicates an amorphous portion and a space indicates a crystal portion, or a mark indicates a crystal portion and a space indicates an amorphous portion. In the case where the optical disk is a magnetic memory medium, a mark indicates an upward magnetization and a space indicates a downward magnetization, or a mark indicates a downward magnetization and a space indicates an upward magnetization. Alternatively, in the case where the optical disk is a magnetic memory medium, a mark may indicate a rightward magnetization and a space may indicate a leftward magnetization, or a mark may indicate a leftward magnetization and a space may indicate a rightward magnetization. In the case where the optical disk is a write-once disk such as a CD-R, a mark indicates a dye burned area and a space indicates a non dye burned area.
The focusing error signal and the tracking error signal are each added to the actuator 107. The position of the objective lens 104 is adjusted so that the beam 70 emitted by the light source 101 is focused at a desired position on the optical disk 105. The methods for generating a focusing error signal and a tracking error signal are well known and thus will not be described here.
FIG. 46 shows a signal processing section 703 including the light detector 158. The electric signal from the light detector 158 is input to the signal processing section 703. As shown in FIG. 46, the light detector 158 includes four light detection sections 158A, 158B, 158C and 158D. The signals from the light detection sections 158A through 158D are respectively current/voltage converted by current/voltage converters 855 through 858. The signals from the current/voltage converters 855 through 858 are sent to an operation section 871 for a differential operation. The signal from the operation section 871 is output from a terminal 814. The signal from the terminal 814 is an inclination detection signal.
In the case where an inclination is detected by the above-described conventional inclination detection apparatus utilizing that eclipse of the beam 70 reflected by the optical disk 105 occurs by the aperture diaphragm of the objective lens 104, the detection sensitivity reduces as the numerical aperture of the objective lens 104 increases. Recently, a structure has been proposed in which the numerical aperture of the light collection system is 0.6 and the thickness of the information memory medium is 0.6 mm in order to increase the information which can be stored in one information memory medium. In such a structure, a mere about 0.5 degree change in the angle made by the beam collected by the objective lens and the information memory medium significantly changes the jitter characteristics of the information read from the information memory medium. In the case where an inclination servo for compensating for the change in the angle made by the beam collected by the objective lens is introduced, the inclination detection apparatus needs to detect the inclination with an error of 0.5 degrees or less. However, in the conventional inclination detection apparatus, when the numerical aperture of the objective lens is 0.6, even if the inclination is actually, for example, 0.5 degrees, the inclination detection signal changes only by about 2%. Thus, it is difficult to precisely detect the inclination of 0.5 degrees or less.
As a third example comparative to the present invention, another conventional optical head device will be described with reference to FIG. 47.
A linearly polarized scattering beam 70 emitted by a semiconductor laser 101 as a light source is collimated by a collimator lens 103 and then is incident on a polarizing beam splitter 130. The beam 70 is transmitted through the polarizing beam splitter 130 and then through a ¼-wave plate 122 to be converted into a circularly polarized beam. The circularly polarized beam is collected on an optical disk 105 by an objective lens 104. The beam 70 reflected and diffracted by the optical disk 105 is again transmitted through the objective lens 104 and then through the ¼-wave plate 122 to be converted into a linearly polarized beam which travels in a direction perpendicular to the direction of the light emitted from the semiconductor laser 101. The beam 70 is then entirely reflected by the polarizing beam splitter 130 and converted into a converged beam (still indicated by reference numeral 70) by a detection lens 133. The converged beam 70 is transmitted to the planar polarizing plate 134 and received by a light detector 158. The beam 70 is provided with a non-point aberration for focusing error detection when passing through the planar polarizing plate 134. The beam 70 received by the light detector 158 is converted into an electric signal in accordance with the light amount thereof.
FIG. 48 shows a signal processing section 705 including the light detector 158. The electric signal from the light detector 158 is input to the signal processing section 705. As shown in FIG. 48, the light detector 158 includes four light detection sections 158A, 158B, 158C and 158D. The signals from the light detection sections 158A through 158D are respectively current/voltage converted by current/voltage converters 851 through 854. The signals from the current/voltage converters 851 and 854 are added together by an addition section 891, the signals from the current/voltage converters 852 and 853 are added together by an addition section 892, the signals from the current/voltage converters 851 and 853 are added together by an addition section 893, and the signals from the current/voltage converters 852 and 854 are added together by an addition section 894. The signals from the adding sections 891 and 892 are sent to an operation section 871 for a differential operation, and the signals from the adding sections 893 and 894 are sent to an operation section 872 for a differential operation. The signal from the operation section 871 is output from a terminal 811, and the signal from the operation section 872 is output from a terminal 812. The signal output from the terminal 811 is a tracking error signal, and the signal output from the terminal 812 is a focusing error signal. The focusing error signal is generated by a well known method referred to as the “non-point aberration method”, and the tracking error signal is generated by a well known method referred to as the “push-pull” method. The focusing error signal and the tracking error signal are respectively added to an actuator 107 for focusing control and another actuator 107 for tracking control. The position of the objective lens 104 is adjusted so that the beam 70 from the semiconductor laser 101 is focused at a desirable position on the optical disk 105.
FIG. 49 shows a structure of the optical disk 105 (FIG. 47). In FIG. 49, Gn−1, Gn, Gn+1, . . . each represent a guide groove for allowing tracking error signal detection. Information is stored in and between the guide grooves as a mark or a space, where a space between two adjacent guiding grooves is Gp and a space between two adjacent tracks is tp, Gp=2·tp.
In the optical head device described as the third example, the following conditions, for example, are adopted in order to store a great amount of information in the optical disk 105. The wavelength λ of the beam 70 from the semiconductor laser 101 as the light source is 650 nm, the numerical aperture NA of the objective lens 104 is 0.6, the thickness t of the optical disk 105 is 0.6 mm, the distance Gp between centers of two adjacent guiding grooves is 1.48 μm, and the distance tp between centers of two adjacent tracks is 0.74 μm. When the angle made by the beam 70 collected by the objective lens 104 and the optical disk 105 is a proper angle, the tracking error signal zero-crosses when the center of the guiding groove is irradiated by the beam 70 collected by the objective lens 104. However, when the angle made by the beam 70 collected by the objective lens 104 and the optical disk 105 is not a proper angle, the tracking error signal does not zero-cross when the center of the guiding groove is irradiated by the beam 70 collected by the objective lens 104. At this point, the tracking error signal is hardly offset but is phase-shifted. Such a phase shift can be a cause of an off-track. For example, when the phase shift is about 0.5 degrees, a 0.1 μm off-track is caused. When the off-track is caused, the information stored in the optical disk cannot be accurately read or erased.